Prescription eyeglass lenses are curved in such a way that light is correctly focused onto the retina of a patient's eye, improving vision. Such lenses are formed from glass or plastic lens “blanks” having certain desired properties to provide the correct prescription for the patient. The lens blanks are usually circular and of substantially larger dimension, for example four inches in diameter and one and a half inches thick, compared to the relatively smaller finished lenses assembled into eyeglass frames. Therefore, a lens blank must be edged to fit an eyeglass frame selected by the patient.
Ophthalmic laboratory technicians cut, grind, edge, and finish lenses according to prescriptions provided by dispensing opticians, optometrists, or ophthalmologists. The specifications include the patient's full prescription, including: 1) the total power the finished lens must have; 2) the strength and size of any segments, if needed (i.e. multifocal lenses); 3) the power and orientation of any cylinder curves; and 4) the location of the optical center and any inducted prism that may be needed. The optical center is the spot on the eyeglass lens where light passes through without refracting, and is normally placed in front of the patient's pupil.
The technician selects a lens blank based on the prescribed power. The selected blank may also include prescribed segments. In order to match the prescription exactly, one ore more curves may be generated or otherwise formed on the back of the blank. Once the curves are formed, a series of dots are drawn on the front surface of the lens to define a 180-degree reference line. If a segment lens is prescribed, the segment edge is parallel to the 180-degree line.
With segment lenses, the distance optical center of the lens is usually slightly above the upper segment edge, and dots are marked at the appropriate distance. The lens may then be aligned and fit onto the edger to edge the required shape onto the lens blank.
At this point, the lens blank has had additional curves formed in the back of the lens, and it has been polished. However, the large diameter blank still has to be sized and shaped to fit into the frame selected by the patient. The lens blank is shaped on a linear lathe, or edger, using a grinding wheel or a router. The lens must again be prepared to accept a block. A lens block is placed where the geometrical center (i.e. the geographic center of the lens that is edged for a frame) of the finished lens will be, and the lens is again oriented on the 180-degree axis. Alternately, the lens block may be placed on the optical center or other major reference point of the lens blank. The lens is then edged to fit the frame, and prepared and assembled with the corresponding frame parts into a finished pair of glasses.
The process of grinding and edging eyeglass lenses from lens blanks requires great care and precision. It is particularly important that precision and accuracy be used when orienting the blank before edging, as described above, so that the proper curvature, optical center and shape are achieved. If the blank is misaligned, the lens will not have the prescribed specifications. Technician time, as well as materials, will be wasted if the blank is misaligned and improperly ground and edged.
Therefore, in order to properly align the lens blank, reference marks are provided on the blank. Blanks are formed with an optical center through which the visual axis of the eye should pass. As noted above, the optical center is usually positioned in front of the pupil. The visual axis is an imaginary line of sight which exits the eyeball through the center of the pupil. Generally, reference marks on each blank define the optical center or major reference point and the 180-degree axis line. In this way, the reference marks define the point through which the visual axis should pass when the line-of-sight of the patient is in the normal primary straight-ahead position. Therefore, it is important that the reference marks on the blank are properly aligned so that an accurate optical center is achieved when the blank is edged. The reference marks on the blank should be on the visual axis when the line-of-sight is in the normal primary straight-ahead position.
In the case of a single vision or monofocal lens, the intersection with the visual axis is usually at the optical center of the lens, which is generally coincident with the geometrical center. A segment or multifocal lens has a defined area(s) with a different prescription. A visible demarcation line separates the different prescriptions. Blanks are formed with these specialized segments, and the upper edge of the segment defined by the demarcation line may be used as a reference to orient the point of intersection of the lens by the visual axis. Likewise, an executive segment lens has a demarcation line which may also be used as a reference for orientation of the point of intersection. The demarcation line on an executive segment lens, however, extends across the entire finished eyeglass lens, as compared to the defined area on the segment lens.
A progressive lens provides additional prescriptive power via a gradual increase in power, which is facilitated by a transition zone. However, progressive lenses do not have demarcation lines, as in segment or executive segment lenses. In a progressive lens, the visual axis for both eyes is oriented at the optical center where the progressive power begins to change on the lens.
Progressive lens blanks usually have two sets of reference marks, a temporary set and a permanent set. The temporary reference marks usually consist of a fitting cross, which falls 2 to 3 mm from the center of the pupil, and a distance reference center mark and a near reference center mark to use in checking the powers of the distance and near prescription areas. The permanent reference marks establish the horizontal line at the optical center of the blank.
Lens blanks generally have reference marks identifying the optical center or other major reference point and the 180-degree axis line on one surface, though these reference marks may differ depending on the lens type. The optical center and 180-degree reference line of the lens are aligned with the center point and 180-degree line of the block, respectively. Edging the blank to a desired peripheral outline involves locating and aligning the optical center and 180-degree reference line with the center point and 180-degree reference line of the block. The periphery of the lens is then edged to a desired shape.
The orientation and positioning of reference marks on a lens blank to the lens block is known in the art. However, there is a degree of inaccuracy in prior art methods because, generally, the reference marks on the blank are visually aligned by the technician with the center point and 180-degree line of the block when securing the lens to the block. Among the drawbacks associated with this general method of alignment is the necessity for the alignments to be made by a skilled technician. Thus, increased labor costs may be incurred. Furthermore, the operation of the blocking equipment by various technicians may affect quality and consistency if various technicians operate the machinery.
Even if a technician is properly trained and skilled, it is often difficult to align the reference marks on the blank with the corresponding block because references marks are not uniform. The size, length and/or width of reference marks vary from blank to blank and laboratory to laboratory. Furthermore, marking device inconsistencies may also result in abnormal or uneven reference marks. Reference marks may also vary depending on the individual actually marking the blank (i.e. the manufacturer or dispensing optician). Thus, methods to align the blank do not provide the accuracy required for precision alignment.
Therefore, there is a need for an alignment system that increases the accuracy and precision of any technician when aligning the blank to the block, or aligning the blank for grinding or edging, regardless of the technician's skill, and regardless of the uniformity of the reference marks on the blank.